They are very situationally dependant. If you are doing something where you might say stop, but not actually mean stop then you need a word that is code for "I need this action to cease now" without spoiling the fun.

Most people can get through their lives without ever needing a safe word, for sex. In anything but a niche situation stop means stop, no means no.

But if you're doing something like some dom/sub thing where the sub is not allowed to say no, then saying it will just make things "worse" for the sub, which can be all part of the fun and games. But you might need the dom to stop, so you have a code word that will bring things to a halt.

You can use a traffic light system where one code word means this is heading to territory I am uncomfortable with so tone it down, but don't stop and the stop-stop. A stop-stop safe word means thing come to an abrupt halt. This can include things like destroying BDSM gear to release the person who called a stop and getting them out of whatever situation ASAP. As an example I had to use my rescue knife to cut a brand new rope my wife and I were using recently because something went wrong and she was in real pain, not just tied up. While that isn't the same as a stop-not-meaning-stop safeword they can be used for myriad reasons.

My wife and I have a set of words for normal every day situations that means get-me-out-of-here that either can invoke when needed.